1. Field of the Invention
This invention relates to a solid electrolytic capacitor having good capacitor characteristics, particularly good high frequency characteristics. The invention also relates to a method for the manufacture of the capacitor mentioned above.
2. Description of the Prior Art
In recent trends toward digitalization of electric and electronic appliances, there is a strong demand of capacitors which exhibit a low impedance in a high frequency range and are small in size and large in capacitance.
Known capacitors for high frequency service include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. Among them, the film and mica capacitors are so large in size that a difficulty is encountered in obtaining large capacitance. With the layer-built ceramic capacitors, temperature characteristics become poorer when they are manufactured to have a smaller size and a larger capacitance coupled with another disadvantage that such capacitors become very expensive.
On the other hand, a certain type of capacitor is known as having large capacitance. Such a capacitor includes, for example, an aluminium dry electrolytic capacitor and an aluminium or tantalum solid electrolytic capacitor. These electrolytic capacitors are advantageous in that since an anodized film serving as a dielectric layer can be formed very thinly, a large capacitance can be imparted to the capacitors. On the contrary, the anodized film is liable to damage, so that it becomes necessary to provide an electrolyte between the anodized film and a cathode in order to repair the defects. With aluminium dry electrolytic capacitors, anode and cathode aluminium foils which have been etched, respectively, are convolutely wound through a paper separator and a liquid electrolyte is impregnated in the separator. This is disadvantageous in that the electrostatic capacitance decreases and the loss (tan .delta.) increases owing to the leakage or evaporation of the liquid electrolyte and in that high frequency characteristics and low temperature characteristics are not good due to the ionic conductivity of the electrolyte.
In order to overcome the disadvantages of the aluminium dry electrolytic capacitors, manganese dioxide is used as a solid electrolyte in the aluminium or tantalum solid electrolytic capacitors. This solid electrolyte is obtained by immersing an anode element in an aqueous manganese nitrate solution and thermally decomposing the nitrate at a temperature of from 250.degree. to 350.degree. C. Since the electrolyte is solid, the drawbacks such as a lowering of the performance caused by the effluent of the electrolyte at high temperatures or by coagulation of the electrolyte at low temperatures can be avoided. The solid electrolytic capacitors exhibit better frequency and temperature characteristics than capacitors using liquid electrolytes. However, such solid electrolytic capacitors are undesirably higher by one or more orders of magnitude with respect to impedance or tan .delta. in a high frequency range than layer-built ceramic capacitors or plastic film capacitors. This is because of the damage of the oxide film during the thermal decomposition of manganese nitrate and the high specific resistance of the resultant manganese dioxide.
In order to solve the above problem, there has been proposed use of organic semiconductors having good anodizability such as a 7,7,8,8-tetracyanoquinodimethane complex (hereinafter referred to simply as TCNQ complex). The organic semiconductor may be impregnated between a counter electrode and the oxide film after dissolution in organic solvent or melting by heating. Accordingly, the damage of the oxide film as will occur during the thermal decomposition of manganese nitrate can be prevented. The TCNQ complexes have high conductivity, good anodizing properties and good high frequency characteristics, with the possibility of making a capacitor with a large capacitance.
For instance, Japanese Laid-open Patent Application No. 58-17609 describes the use of a solid electrolyte which is made of an organic semiconductor comprised of N-n-propyl or N-isopropylisoquinoline and TCNQ complexes. According to this patent application, the impregnation of the TCNQ salt in a convolutely wound aluminium electrolytic capacitor is carried out by application of a melt of the TCNQ salt, thereby firmly bonding the TCNQ salt and the oxide film together. The resultant aluminium capacitor is stated as being significantly improved in frequency and temperature characteristics owing to the high conductivity of the TCNQ salt. Since the TCNQ salt has higher conductivity and better anodizing ability (repairing ability) than manganese dioxide, one is enabled to make a capacitor whose frequency and temperature characteristics are better than solid electrolytic capacitors using manganese dioxide. The TCNQ salts used in the patent application contain an isoquinolium cation which is substituted with an alkyl group at the N position. This salt is impregnated in the oxide film after heat melting.
In recent years, there have been proposed solid electrolytic capacitors wherein highly conductive polymers obtained by polymerizing heterocyclic monomers such as pyrrole, thiophene and the like are formed on an anode for used as an electrolyte (Japanese Laid-open Patent Application Nos. 60-37114 and 61-47625).
According to electrolytic polymerization, a dense conductive polymer film may be readily formed, on an ordinary anode such as platinum, carbon or the like, from a solution of pyrrole, thiophene or derivatives thereof and an appropriate support electrolyte. However, the formation of a conductive polymer on an anode having an oxide film by the electrolytic polymerization is principally difficult since any electric current does not pass. Although a conductive polymer film may be formed on the oxide film-free surface of a valve metal by the electrolytic polymerization, anodic conversion treatment through the conductive polymer film is essential. This will cause the previously formed conductive polymer film to be changed in quality or deteriorate, or may be separated from the anode surface, thus leading to a lowering of the characteristics of the capacitor.
The formation of a conductive polymer may be possible by the use of oxidation polymerization. The resultant polymer is in the form of powder and is poor in adhesion to an anode having an oxide film. Accordingly, such a polymer is difficult to utilize as an electrolyte for capacitor.